Collier finds hollow alluvial rock (7)
I believe the answer is:
mineral
'rock' is the definition.
'collier finds hollow alluvial' is the wordplay.
'collier' becomes 'miner' (I've seen this before).
'finds' is a charade indicator (letters next to each other).
'hollow' says to hollow out the word (remove centre letters) (the word is hollowed out).
'alluvial' with its middle taken out is 'al'.
'miner'+'al'='MINERAL'
(Other definitions for mineral that I've seen before include "Substance needed for health; ore" , "Ore; soft drink" , "What drink is available in ground" , "Gypsum or magnesium, say" , "Gypsum or magnesia, say" .)